Method of manufacturing plated hollow blocks

ABSTRACT

A method of manufacturing plated hollow metallic blocks for further processing into seamless tubes of the type in which a body is immersed one or more times into a melt includes: protecting the inner surfaces of a hollow body of plating material against the admission of a melt of support material during the immersion of the hollow body in the melt of support material; immersing such hollow body formed of plating material into the melt of support material; removing the hollow body from the melt of support material; and crystallizing a layer of support material on the outer surface of the hollow body.

FIELD OF THE INVENTION

The present invention relates to a method of manufacturing plated hollowblocks which can be further processed by hot or cold working intoseamless metal tubes, such as steel tubes, which are plated at theinside thereof.

BACKGROUND OF THE INVENTION

The manufacture of internally plated seamless steel tubes has up to nowbeen generally performed in the manner that a hollow block formed of asupport material and a plating material is shaped by extrusion so as toform a tube. Initially, a cylindrical block of a support material suchas low-alloy steel is drilled in an axial direction so as to create ahollow block. A cylindrical block of a plating material such as highalloy steel and of the same length and of a diameter corresponding tothe inside diameter of the hollow block formed of the support materialis also drilled in the axial direction and inserted into the hollowblock of said support material.

The two hollow blocks are placed one within the other and are weldedtogether at their ends so that the annular slot between the two hollowblocks is tightly closed off so that the contact surfaces of the hollowblocks do not oxidize upon heating to the extrusion temperature andprevent a suitable bond between the support material and the platingmaterial.

However, this procedure has serious disadvantages. The weld on the endsrepresents a weak point which can tear, for instance, upon heating sothat the contact surfaces may nevertheless oxidize. In addition, thepreparation of a plated hollow block which is ready for use requiresconsiderable expense on the one hand, due to the working (i.e.,drilling, welding) necessary and, on the other hand, the considerableuse of expensive plating material (i.e., waste produced upon thedrilling).

For the manufacture of a steel plate which is plated on one side, theapplicant has already proposed a method (Federal Republic of Germany P39 07 903) in which the plating material is applied in molten state to asupport plate. For this purpose, the flat sides of two support platesare placed tightly on one another and dipped into a melt of the platingmaterial until a sufficiently thick layer of plating has formed bycrystallization.

However, this procedure of applying a layer of plating directly from themolten state onto the support material cannot be directly applied to themanufacture of plated hollow blocks. Upon the immersion of a hollowblock of the support material into a melt of the plating material, aplating layer would immediately form on the inner surface as well as theouter surface. This outer layer is not necessary and would greatlyincrease the cost of manufacture due to the unnecessary consumption ofplating material. In order to avoid outside plating, it is possible tofill a hollow block of support material with a melt of the platingmaterial or, in order to keep the consumption of plating material assmall as possible, centrifuge it, for instance, with a layer of thismaterial and allow it to solidify. In such a case, however, the problemarises that due to different thermal expansion and shrinkage, theplating layer detaches itself from the support material before thefurther processing of the plated hollow block can take place.

SUMMARY OF THE INVENTION

The object of the present invention, therefore, is to provide a methodby which it is possible to produce a hollow block which is plated on theinside and which avoids the disadvantages indicated.

The object of the present invention is achieved by a method wherein theinner surface of a hollow body is protected against the admission of amelt of support material. The hollow body is composed of a platingmaterial and is dipped into a melt of support material and then removedfrom the melt of support material so as to crystallize a support layeron the outer surface of the hollow body.

The solution in accordance with the invention contemplates that themolten support material be applied to the outside of the solid platingmaterial. In this way, assurance is had from the very start that theinner layer of plating cannot detach itself from the outer layer as aresult of thermal shrinkage since the outer layer in any event tendstowards greater shrinkage as a result of its higher initial temperatureso as to remain on the plating layer. The cylindrical hollow body usedfor the crystallization of the layer of the support material can beproduced, for example, by the hot working of a corresponding block in apunch press and insofar as necessary, machined mechanically on theinside and outside before it is immersed into the melt of supportmaterial in order to obtain clean and smooth surfaces.

Thus, it is possible to manufacture a cylindrical hollow body which isfree of or requires little if any machining, and results in minimalwaste of plating material. The sealing of the inner surface of thishollow body during the immersion in the molten support material can beachieved, for example, by a closure cover. However, it is preferred thata cylindrical core be used for this purpose which rests tightly againstthe inner surface of the hollow body. It is particularly advantageous ifthe core used in sealing the surface of the hollow body, is already usedin producing the cylindrical hollow body by dipping the core into a meltof the plating material and allowing the required layer of platingmaterial to crystallize thereon. For this purpose, the core must consistof a sufficiently heat-resistant material, for instance a structuralsteel. Its heat resistance need merely permit the core to be dipped forthe required time into the melt without itself melting. For thispurpose, it is advisable to provide the core with internal cooling, forexample, by conducting a coolant through it. For easy removal of thecore from the hollow body or block to be possible, the core must beprovided on its outer surface with a parting or separating layer whichis effective with respect to the melt. In the case of a steel core, alayer of rust or scale is, for instance, suitable for this purpose. Thisprevents direct connection between the plating material and the materialof the core and makes it possible to withdraw the core from the hollowbody.

The required dwell time of the steel core in the molten plating materialdepends upon whether a separate internal cooling of the core has beenprovided and upon the heat absorption capacity of the core. In order tocrystallize thicker layers, the step of dipping the melt can beperformed in partial steps, in which case an intermediate cooling stepis performed before the next dip occurs in the melt. This procedure canbe used both for the production of the plating layer and for theproduction of the support layer.

If the surfaces which are produced by the crystallizing of the platingmaterial and/or of the support material are too irregular, smoothing ofthe surfaces can be performed, at minimal expense, by rolling while thematerial is still in the hot state. If the manufacture of the hollowbody consisting of the plating material has been performed by dipping acore of heat-resistant material, having a parting layer on its surface,into a melt and then removing it from the melt so as to crystallize alayer of plating material on the surface of the core, then a mechanicalmachining so as to obtain a clean and smooth inner surface, ispreferably performed before processing the hollow block into a seamlesstube. In this case, only a small amount of waste material is produced.Further processing can be performed, for example, by extrusion while inthe hot state or else by hot or cold pilger processing. The method ofthe invention is, in particular, suitable for steel materials, but itcan also be used for other types of metallic materials.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described in further detail with reference tothe following two examples for the manufacture of seamless, internallyplated steel tubes of St37.

A tube of a length of about 1 meter, outside diameter 120 mm and 30 mmwall thickness, of plating material 1.4301 (X5 CrNi 18 9) closed by acover was dipped for about 25 seconds into a melt of steel of the gradeSt37, heated about 20° K. above liquidus temperature and then removedfor intermediate cooling to about room temperature. During the time ofimmersion a layer of St37 of a thickness of about 22 mm crystallized onthe outside of the tube. This immersion process, followed byintermediate cooling, was repeated two more times until obtaining ahollow block of a total outside diameter of 252 mm. The outer surface ofthe hollow block was then smoothed in hot condition by sizing rolling.

The immersion time selected for the production of the hollow block wasselected so as to permit the greatest possible rate of growth of thesupport material St37 and, to permit an excellent connection between theplating layer and the support material. The hollow block produced wasthen extruded in a known manner while hot in an extruder to form aseamless steel tube of a length of about 21 meters, an outside diameterof about 80 mm, and a wall thickness of about 10 mm. The plating layerhad a thickness of about 2 mm and was flawlessly attached to the supportmaterial.

In a second example, a hollow block of about 250 mm outside diameter, 60mm inside diameter, a thickness of the plating layer of about 25 mm, anda length of about 1 meter was produced and shaped into a seamless tube.For this purpose, a bar of St37, of an outside diameter of about 60 mm,which was covered with a layer of scale was dipped into a melt ofplating material 1.4301 and heated to about 30° K. above liquidustemperature. After an initial immersion time of about 35 seconds, duringwhich a plating layer of about 17 mm had formed on the surface, the barwas removed from the melt. After an intermediate cooling to about roomtemperature, it was again dipped into the melt of plating material inorder to obtain a total thickness of the plating layer of about 25 mm.To accomplish this, the immersion time was increased to about 47seconds, i.e. one waited until the second plating layer which had grownand had reached its maximum in about 35 seconds, had partially meltedoff again. An immersion time of less than 35 seconds to obtain the 8 mmstill missing from the desired thickness of the layer would have beenunsuitable since the adherence to the first plating layer would thenhave been insufficient. After intermediate cooling, the bar providedwith a plating layer of a thickness of 25 mm was then dipped inaccordance with the first embodiment into a melt of St37 and heated toabout 20° K. above liquidus temperature.

After three dippings and intermediate coolings, a block of an outsidediameter of 236 mm had formed. In order to reach the desired outsidediameter of 250 mm, a final dip of a duration of 53 seconds was thenperformed. Upon removal from the melt and complete solidification of theouter surface, the rod of St37 used as immersion core was pulled out ofthe hollow block on a removal device. Because of the layer of scaleacting as parting or separating layer on the rod, this separation couldbe achieved without difficulty. The outer surface of the block was thensmoothed while still hot. The inner surface (plating layer) of thehollow block was also subjected to a smoothing and cleaning operation inorder to eliminate the irregularities caused by the layer of scale. Theblock was then again shaped hot in an extruder to form a seamless tube.With an outside diameter of 80 mm and an inside diameter of 30 mm, therewas obtained a tube length of more than 20 meters with a thickness ofthe plating layer of 1.6 mm. The attachment between the two layers wasagain flawless.

It should be understood that the preferred embodiments and examplesdescribed are for illustrative purposes only and are not to be construedas limiting the scope of the present invention which is properlydelineated only in the appended claims.

We claim:
 1. A method of manufacturing an internally plated hollowmetallic block of the type which can be further processed into aninternally plated seamless steel tube, comprising:(a) forming acylindrical hollow body of plating material, said hollow body having abore and in inner and outer surface; (b) protecting said inner surfaceof said hollow body against the admission of melted material; (c)immersing said hollow body formed of plating material into a melt ofsupport material; and (d) removing said hollow body from said melt ofsupport material after a layer of support material has crystallized onthe hollow body of plating material.
 2. The method according to claim 1,wherein the step of protecting said inner surface of said hollow bodyincludes inserting a core in said bore of said hollow body so as to resttightly against said inner surface of said hollow body and furthercomprising the step of removing the core from said hollow body aftersaid layer of support material has crystallized on the hollow body. 3.The method according to claim 2, wherein said hollow body of platingmaterial is formed by:(a) immersing a core, having an outer surface,being composed of heat-resistant material and having a parting layer onsaid outer surface, into a melt of said plating material; (b) removingsaid core from said melt of plating material after a layer of platingmaterial has crystallized on the core; and (c) removing said core fromsaid hollow body of plating material.
 4. The method according to claim2, wherein said crystallization of said support material is performed inat least two steps by removing said hollow body from said melt ofsupport material after a period of immersion within said melt of supportmaterial; and by subjecting the hollow body to an intermediate coolingperiod before again immersing said hollow body in said melt of supportmaterial so as to permit further growth of the crystallized layer ofsupport material.
 5. The method according to claim 3, wherein saidcrystallization of said plating material is performed in at least twosteps by removing said core from said melt of plating material after aperiod of immersion within said melt of plating material; and bysubjecting said core to an intermediate cooling period before againimmersing said core in said melt of plating material so as to permitfurther growth of the crystallized layer of plating material.
 6. Themethod according to claim 1, further comprising the step of smoothingsaid surface of plating material prior to immersing said body into saidmelt of said support material.
 7. The method according to claim 1,further comprising the step of smoothing the outer surface of saidsupport material before said hollow block undergoes further processing.8. The method according to claim 6, wherein said step of smoothing isperformed by a smoothing roll.
 9. The method according to claim 7,wherein said step of smoothing is performed by a smoothing roll.
 10. Themethod according to claim 3, further comprising the step of cooling theinside of said core by a stream of coolant during the immersion of saidcore into said melt of plating material.
 11. The method according toclaim 3, further comprising the step of cooling the inside of said coreby a stream of coolant during the immersion of said core and said hollowbody into said melt of support material.
 12. The method according toclaim 3, further comprising the steps of cleaning said inner surface ofthe hollow block after removing said core from said hollow block; andsmoothing said inner surface of said block before further shaping isperformed on said block.